NEW FRONTIERS IN COSMIC CARBON – Brett A. McGuire
(Massachusetts Institute of Technology)
The detection of the aromatic molecule benzonitrile (C6H5CN) in the cold, dark, starless
interstellar cloud TMC-1 has opened a new window onto a previously disregarded regime of carbon chemistry at
the earliest stages of star formation. Recent work by the GOTHAM collaboration has revealed that benzonitrile
is only the tip of the proverbial molecular iceberg in TMC-1, hinting at a new source for interstellar polycyclic
aromatic hydrocarbons (PAHs). PAHs, by some accounts, are a reservoir of as much as 25% of all interstellar carbon,
making a thorough understanding of their formation and evolution essential to unraveling the tangled web of cosmic
molecular evolution. Our work has also shown that aromatic carbon chemistry – including PAHs – is
apparently ubiquitous throughout the early star-formation cycle and into the protostellar phase. Here, I will
discuss the extent of this previously hidden reservoir of carbon and its potential implications on the chemistry
of forming star and planetary systems.
ANHARMONIC FREQUENCIES FOR THE DETECTION OF LARGE MOLECULES IN SPACE – Brent Westbrook
(University of Mississippi)
Quartic force fields (QFFs) offer highly accurate anharmonic rovibrational spectroscopic constants. The
state-of-the-art composite approach is known as CcCR and often achieves accuracies within 1 cm–1
of gas-phase experiment for fundamental frequencies and 20 MHz for principle rotational constants. The performance
of explicitly-correlated methods is nearly as good, offering accuracies of 5-7 cm–1 at a much
lower computational cost. However, both of these QFF approaches typically rely on a complex internal coordinate
system that can be difficult to derive for large or highly symmetric molecules. Extending this methodology to use
generic Cartesian coordinates and the analytic derivatives found in many popular quantum chemistry packages will
allow the elucidation of such spectral data for larger molecules. Preliminary work on ammonia borane and other
even larger molecules demonstrates the efficacy of this new approach.
THE UNSOLVED ISSUE WITH OUT-OF-PLANE BENDING FREQUENCIES FOR C=C MULTIPLY BONDED SYSTEMS– Timothy J. Lee
(NASA Ames Research Center)
More than 30 years ago two groups independently identified a problem in the calculation of the out-of-plane bending
(OPB) vibrational frequencies for the ethylene molecule using correlated electronic structure methods. Several studies
have been done in the meantime to try and understand and resolve this issue. In so doing this problem has been found
to be far more insidious than previously realized for acetylene-like and benzene-like molecules, which can become
non-linear and non-planar, respectively. The one common feature that all molecules with this problem have is that
they contain C=C multiple bonds, and so this has been called the "C=C multiple bond OPB frequency issue" or "the
C=C OPB problem." Various explanations for this problem have been advanced such as basis set superposition error,
basis set incompleteness error, linear dependences in the basis set, proper balancing of the basis set between
saturation and inclusion of higher angular momentum functions, etc. and possible solutions have arisen from these
suggestions. All of these proposed solutions, however, amount to one main point connecting them all: modifying the
one-particle basis set in some way. None of the explanations that have been advanced, however, really fit all of the
data for all of the molecules where this problem has been identified, and importantly, none of these diagnostic tests
have been applied to similar molecules where this issue does not appear. In this review, the studies over the last 30
plus years are discussed and relevant data from each of these is compared and contrasted. Recent density functional
theory studies on polycyclic aromatic hydrocarbons, that may or may not be connected to this issue, are also discussed
as well as the implications on computing anharmonic vibrational spectra in order to compare with astronomical observations.
It is hoped that by collecting and analyzing the data from these studies a path forward to understanding and resolving
this issue will become evident.
THE FIRST MID-INFRARED DETECTIONS OF HNC and H13CN IN THE INTERSTELLAR MEDIUM – Sarah Nickerson
(NASA Ames Research Center)
We present the first mid-infrared (MIR) detections of HNC and H13CN in the interstellar medium, and
numerous HCN transitions. Our observations span 12.8 to 22.9 μm towards the hot core Orion IRc2, obtained with the
Echelon-Cross-Echelle Spectrograph aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA/EXES). 5 km/s
resolution distinguishes individual rovibrational transitions of the three molecules, allowing direct measurement of
their excitation temperatures, column densities, and relative abundances. HNC and HM13MCN share temperatures
of 100 K with a local standard of rest velocity of -7 km/s. HCN shows two velocity components at -7 km/s at 165 K,
and 1 km/s at 309 K. The -7 km/s velocity component measured for all three molecules is similar to an outflow from
the nearby high mass protostar Radio Source I, and are likely associated with it. The 1 km/s component is the hottest
measured HCN to date towards IRc2 and closest to the hot core’s centre. EXES's smaller beam size compared to most other
detections allows us to focus on the hot core itself without confusion from surrounding sources. Previous observations
at longer wavelengths detected colder components of these three molecules in emission, while the MIR observations are
hotter and in absorption. We utilize a gas-grain chemical network to model the HCN/HNC evolution, which reaches our
derived HCN/HNC=72 after 106 years. This is much older than the region's explosive event 500 years ago,
suggesting that the hot core's origins predate this event. Our derived 12C/13C=13 is lower
than measurements at longer wavelengths. Several other recent observations towards star-forming regions also show
similarly unexpectedly low isotope ratios. This points to the possibility that the isotope chemistry in these regions
is not yet fully understood.
COMPUTATIONAL EXPLORATIONS OF SOME NOVEL GROWTH MECHANISMS LEADING TO PAH AND PANH SPECIES – Martin Head-Gordon
(Kenneth S. Pitzer Distinguished Professor of Chemistry, University of California, Berkeley)
There have been significant improvements in the ability of density functional theory (DFT) methods to
accurately model chemical reactions over the past 5 years. These improvements will be summarized, and
the resulting methods applied to address some interesting pathways towards the formation and growth of
aromatics under conditions relevant to the interstellar medium. The reactions considered will include
viable routes to formation of benzene from acetylene clusters or ices under ionizing conditions, and
the corresponding pathways to pyridine (with relevance to the origin of DNA bases) in the presence of
both acetylene and hydrogen cyanide. If time permits, ionization-induced bond formation between aromatics
and pyridine will also be considered to unravel the nature of this exotic strong interaction.
PRESTELLAR PROVENANCE OF COMET 67P/CHURYUMOV-GERASIMENKO – Maria Drozdovskaya
(Center for Space and Habitability, University of Bern)
Our Solar System harbors several thousand known comets with many more awaiting discovery. Comets are thought
to be some of the most pristine relics that survive to this day, which may shed light on the volatiles and
refractories that nourished our infant Solar System. In my talk, I will discuss the results from the ESA
Rosetta mission that were obtained during its two year-long escort of comet 67P/Churyumov-Gerasimenko. I
will address the story told by the comet's volatile inventory and deuteration, as deduced by the ROSINA
instrument. The connections with star-forming regions, such as the low-mass IRAS 16293-2422, will be explored.
In my talk, I will present evidence that leads to the conclusion that comets are capable of revealing the
full evolutionary scenario of the low-mass system that we call home.
THE UNEXPECTED CHEMISTRY IN PLANETARY NEBULAE: FROM CO TO C60 – Lucy M. Ziurys
(Regents Professory, CBC and Astronomy, University of Arizona)
For decades, planetary nebulae (PNe) have been considered to have limited molecular content, confined mostly
to diatomic molecules. The strong ultraviolet radiation field generated by the central white dwarf star was
thought to destroy molecules readily generated in the prior Asymptotic Giant Branch (AGB) phase. Theoretical
calculations verified this line of thought. Over the past several years, however, we have been conducting
observations of various molecules in PNe at millimeter wavelengths. We have now detected polyatomic molecules
such as HCN and HCO+ in over 30 PNe, and species as complicated as CH3CN, H2CO,
and c-C3H2 in several select nebulae. Furthermore, the molecular abundances do not appear
to significantly vary with the age of the nebulae over the 10,000 year PN lifetime. Circumstellar abundances
appear to be principally altered in the protoplanetary nebulae (PPNe) stage. Such molecular material must seed
diffuse clouds, accounting for the polyatomic species observed there. Notably, C60 is also observed
in some PNe. We have also conducted recent solid-state laboratory imaging and spectroscopy, as well as molecular
dynamics (MD) simulations, that suggest that C60 is formed in the PPNe phase from the destruction of
silicon carbide (SiC) grains.
ASTROCHEMICAL FORECASTING WITH MACHINE LEARNING –
Kelvin Lee, (Department of Chemistry, Massachusetts Institute of Technology)
Since the first molecules were detected in space, we have now reached a point where chemical and physical
complexity in the interstellar medium reaches the boundaries of what human expertise and intuition alone can
achieve. With every new molecule we discover, the question "What comes next?" grows more and more difficult to
answer as more possibilities emerge. Conventionally, we turn to chemical models for guidance; this may be
complicated when considering complex, non-LTE processes such as shocks, radiation, and grain-surface chemistry.
Moreover, expansion of chemical networks typically requires hand-picked reactions and species, requiring an
exhaustive knowledge of chemical and astrophysical literature, and can impose human bias on which reactions
and molecules are important. As a complimentary approach to conventional chemical models, we have developed
an unsupervised machine learning pipeline for predicting molecular abundances in a non-parametric fashion.
Leveraging tools originally developed in high throughput drug discovery and data science, our pipeline captures
and uses millions of molecules from various databases to create chemically descriptive vector representations
for quantitative comparison. These representations are subsequently used to predict molecular properties in a
given environment; as a proof-of-concept, we use the well-characterized chemical inventory of TMC-1, including
the latest discoveries from the GOTHAM collaboration. We show that the model can be successfully conditioned
on an inventory, able to reproduce column densities of unseen molecules to within an order of magnitude without
any tuning parameters. Simultaneously, we are able to use the model to predict column densities of hundreds of
thousands of molecules not yet detected in space, as a way to guide efforts, as well as provide a robust
statistical baseline for expected abundances.
PREBIOTIC ASTROCHEMISTRY IN THE "THz-GAP" – Susanna Widicus Weaver (Department of Chemistry, University of Wisconsin - Madison)
Small reactive organic molecules are key intermediates in interstellar chemistry, leading to the formation of
biologically-relevant species as stars and planets form. These molecules are identified in space via their pure
rotational spectral fingerprints in the far-IR or terahertz (THz) regime. Despite their fundamental roles in the
formation of life, many of these molecules have not been spectroscopically characterized in the laboratory, and
therefore cannot be studied via observational astronomy. The reason for this lack of fundamental laboratory
information is the challenge of spectroscopy in the THz regime combined with the challenge of studying unstable
molecules. Our laboratory research involves characterization of astrophysically-relevant unstable species, including
small radicals that are the products of photolysis reactions, organic ions formed via plasma discharges, and small
reactive organics that form via O(1D) insertion reactions. Our observational astronomy research seeks to examine the
chemical mechanisms at play in a range of interstellar environments and to identify chemical tracers that can be used
as clocks for the star-formation process. In this seminar, I will present recent results from our laboratory and
observational studies that examine prebiotic chemistry in the interstellar medium. I will discuss these results in
the broader context of my integrative research program that encompasses laboratory spectroscopy, observational
astronomy, and astrochemical modeling.
PHYSICOCHEMICAL MODELS: SOURCE-TAILORED OR GENERIC? –
Beatrice Kulterer, (Center for Space and Habitability, Universität Bern)
Physicochemical models can be powerful tools to trace the chemical evolution of a protostellar system and allow to
constrain its physical conditions at formation. I will discuss whether source-tailored modelling is needed to explain
the observed molecular abundances around young, low-mass protostars or if, and to what extent, generic models can improve
our understanding of the chemistry in the earliest stages of star formation (Kulterer et al. 2020). The physical conditions
and the abundances of nine simple and abundant molecules based on three models are compared. The physical models considered
are 1D or 2D, the chemical networks consider two or three phases. After establishing the discrepancies between the calculated
chemical output, the calculations are redone with the same chemical model for all three sets of physical input parameters.
With the differences arising from the chemical models eliminated, the output is compared based on the influence of the
physical model. Results suggest that the impact of the chemical model is small compared to the influence of the physical
conditions, with considered timescales having the most drastic effect. Source-tailored models may be simpler by design;
however, likely do not sufficiently constrain the physical and chemical parameters within the global picture of star-forming
regions. Generic models with more comprehensive physics may not provide the optimal match to observations of a particular
protostellar system, but allow a source to be studied in perspective of other star-forming regions.
THE RIDDLE OF COMPLEX ORGANIC MOLECULES – Eric Herbst (Departments of Chemistry and Astronomy, University of Virginia)
In addition to stars, galaxies such as our own Milky Way contain interstellar matter, much of which is condensed
into so-called interstellar clouds consisting of gas and nanoparticles known as dust grains. The clouds, ranging in
size from a few to 100's of light years in extent, are of great interest as the ultimate sites of star and planetary
formation. The denser clouds contain large numbers of molecules in the gas phase, mainly organic in nature, divided
into classes of molecules dependent upon the age and physical nature of the clouds. Molecules are also observed in
ice mantles of cold dust particles. Two distinctive classes of gaseous molecules are known as "carbon chains" and
"complex organic molecules (COMs)". Carbon chains are exotic, very unsaturated, and often linear, whereas COMs
resemble small terrestrial organic solvents, consisting of amines, alcohols, esters, etc. Until recently, based on
observations, it was thought that carbon chains exist solely in cold dense clouds, whereas COMs exist in warmer regions
in which star formation is occurring, known as "hot cores." More recently, COMs have been discovered in cold regions
as well, introducing more complexity into our understanding of their chemistry. This talk will be concerned mainly
with the local build-up chemistry thought to form COMs in both types of sources, and the degree of success that has
been achieved.
FORMATION OF COMPLEX ORGANIC MOLECULES IN THE TRANSLUCENT CLOUD VIA TOP-DOWN PROCESSING ON DUST GRAINS –
Ko-Ju Chuang,* (Max Planck Institute for Astronomy)
Interstellar complex organic molecules (COMs) have been identified toward various star-forming regions from
translucent clouds to the solar system. Interstellar sugar-like species
(CnH2nOn) have been intensively studied along with the bottom-up
approaches; the ice chemistry scheme of
CO-H2CO-CH3OH through "energetic" and/or "non-energetic" processing on dust grains. However,
the icy origin of acetaldehyde and its (de-)hydrogenated derivatives (C2HnO), which are
often observed in molecular clouds before CO freezes out, remains unclear. In this talk, I will present the
laboratory study on the solid-state reactions that involve C2H2, which is one of the common
hydrocarbon fragments of PAHs or hydrogenated carbonaceous dust (HAC) in the top-down scenario, and H/OH-radicals
along with the H2O formation/destruction sequence on grain surfaces under molecular cloud conditions.
It is concluded that C2H2 readily acts as a molecular backbone providing a solid-state route
for the formation of COMs, such as ketene (CH2CO), acetaldehyde (CH3CHO), vinyl alcohol
(CH2CHOH), ethanol (CH3CH2OH), and possibly acetic acid (CH3COOH).
The reaction network linking the above complex species described by the formula C2HnO is present.
*Dr. Chuang received Honorable Mention in the 2020 competition for the Astrochemistry Subdivision
Dissertation Award.
THE MOLECULAR UNIVERSE – Alexander Tielens (Leiden Observatory, Leiden University and Astronomy Department,
University of Maryland)
Over the last 20 years, we have discovered that we live in a molecular Universe: a Universe with a rich and varied organic
inventory; a Universe where molecules are abundant and widespread; a Universe where molecules play a central role in key
processes that dominate the structure and evolution of galaxies; a Universe where molecules provide convenient thermometers
and barometers to probe local physical conditions. Understanding the origin and evolution of interstellar and circumstellar
molecules is therefore key to understanding the Universe around us and our place in it and has therefore become a fundamental
goal of modern astrophysics. The field is heavily driven by new observational tools that have become available over the last
20 years; in particular, space-based missions that have opened up the IR and submillimeter window at an ever-accelerating
pace. Furthermore, our progress in understanding the Molecular Universe is greatly aided by close collaborations between
astronomers, molecular physicists, astrochemists, spectroscopists, and physical chemists who work together in loosely
organized networks. In this talk, I will sketch the progress that we have made over the last 20 years and outline some
of the challenges that are facing us. The focus will be on understanding the unique and complex organic inventory of
regions of star and planet formation that may well represent the prebiotic roots to life.
A PHOTOIONIZATION REFLECTRON TIME-OF-FLIGHT INVESTIGATION OF PHOSPHORUS CHEMISTRY IN EXTRATERRESTRIAL ICES –
Andrew M. Turner,* Cornelia Meinert, Ralf I. Kaiser (University of Hawaii at Manoa)
Multiple phosphorus-containing compounds have been detected in the Solar System (planetary atmospheres, comets,
meteorites) along with interstellar and circumstellar environments. Of particular astrobiological interest are
alkyl phosphonic acids (RH2PO3, R = methyl, ethyl, propyl, and butyl) extracted from the
Murchison meteorite. These phosphonic acids are the only extraterrestrial phosphorus-containing organic compounds
thus far discovered and offer a bioavailable and highly soluble form of phosphorus. This project investigates the
synthesis of phosphorus-containing products of electron-irradiated interstellar ice analogues containing phosphine
(PH3), water (H2O), carbon dioxide (CO2), and hydrocarbons such as methane
(CH4). Phosphine is known to exist in circumstellar envelopes (IRC +10216), has been considered for comets
(67P/Churyumov-Gerasimenko), and may serve as the phosphorus source of complex organic compounds such as the alkyl
phosphonic acids. Utilizing in situ analysis techniques such as quadrupole mass spectrometry (QMS),
tunable-photoionization reflectron time-of-flight mass spectrometry (PI-ReTOF-MS), and infrared spectroscopy (FTIR)
in addition to ex situ analysis by secondary-ion mass spectrometry (SIMS) and two-dimensional gas chromatography mass
spectrometry (GCxGC-TOF-MS), the intermediates and products of these irradiated ice analogues are characterized to
demonstrate the potential to synthesize organic phosphine-containing molecules in astrophysical environments. Notable
results include phosphanes (PxHx+2), methylphosphanes (CH3PxHx+1),
and phosphorus oxoacids (H3POx, x=1–4, and pyrophosphoric acid
(H4P2O7) along with their alkylated equivalents such as prebiotically significant
methylphosphonic acid (CH3P(O)(OH)2) and methylphosphate (CH3OP(O)(OH)2).
*Dr. Turner is the 2020 recipient of the Astrochemistry Subdivision Dissertation Award.
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